This study was performed to evaluate the inactivation and microbial regrowth of heterotrophic and nitrifying bacteria using chloramine as a secondary disinfectant for drinding water distribution system. Three sets of the three reactors filled with the $Cl_2/NH_3-N$ ratio of 3:1, 4:1, and 5:1 were used in these experiments. Chloramine concentration were applied to each set of the reactors with $1mg/ell$,;2mg/ell;and;3mg/ell$, respectively. For the set with elapsed time and reached to zero level after 7 days. Heterotrophic bacteria remarkably increased and nitrification through the experimenatal period (21 day). Furthermore the regrowth of heterotrophic bacteria and nitrification were not found. More than $2mg/ell$ of chloramine with $Cl_2/NH_3-N$ ratio of 3:1, the nitrification could be inhibited by 2 days of contact time.